Wireless access to information systems and the Internet is becoming mandatory for businesses that provide online services. Businesses require that the existing online services and applications be available to any wireless client. Most applications, whether designed to use over wired or wireless networks, rely on the TCP protocol for reliable transport (F. M. Anjuman and L. Tassiulas, “On the Behavior of Different TCP Algorithms Over A Wireless Channel,” 155-165, Atlanta, May 1999. ACM Sigmetrics.) Since these applications are TCP-based, wireless clients need to support some type of reliable transport to interface with these applications seamlessly.
TCP is designed for operation in wired networks, where random packet losses due to transmission errors are negligible. It is well known that TCP performs poorly over wireless links, which suffer from packet losses due to the error-prone and idiosyncratic nature of the wireless medium. TCP does not adapt well to wireless communications, as it interprets the packet losses as being caused by network congestion. In addition, acknowledgement and retransmission schemes used in TCP unnecessarily consume network bandwidth. (H. Balakrishnan, V. N. Padmanabhan, S. Seshan, and R. H. Katz, “A Comparison Of Mechanisms For Improving TCP Performance Over Wireless Links,” IEEE/ACM Transactions on Networking, June 1997.); (R. W. Lucky, “Improving Reliable Transport And Handoff Performance In Cellular Wireless Networks,” ACM Wireless Networks, 14, Dec. 1995.); (R. Caceres and L. Iftode, “Improving Reliable Transport Protocols In Mobile Computing Environment,” IEEE JSAC, 1994.)
Yet another problem arising from the conventional TCP relates to power consumption.
Hand-held devices coupled with wireless network interfaces are emerging as a new way to achieve seamless connectivity while adhering to stringent power and size constraints. The measurement-based results clearly indicate that the power drained by the network interface constitutes a large fraction of the total power used by the PDA. Simulation results show that the predominant cost comes not from the number of packets sent or received by a particular transport protocol, but from the amount of time the network interface is in an idle state. The energy cost can increase significantly in the presence of wireless losses, since a receiver must wait for a TCP sender to recover from packet losses.
The loss-intensive wireless communication results in high retransmission rates and bandwidth consumption. In addition, retransmitted segments have a significant chance of being dropped.
A number of efforts have been made to tune the TCP protocol for wireless medium. Each of them addresses different aspects of the problem and provides solutions specific to that. Major efforts are identified below:
Snoop TCP
This solution addresses the problem associated with TCP congestion control algorithm. When packet loss is detected (duplicate acknowledgement or local timeout), snoop retransmits the lost packet, hiding the fixed host (FH) from duplicate acknowledgements, and thereby preventing congestion control from kicking in. The down side is that each base station has to be updated with the snoop module (R. Caceres and L. Iftode, “Improving Reliable Transport Protocols In Mobile Computing Environment,” IEEE JSAC, 1994).
Wireless Profiled TCP
This particular technique is used in commercially available wireless application protocol and its focus is mostly on “initial window size” adjustment to avoid the burden of the “Slow Start” phase of TCP. This approach introduces “Explicit Congestion Notification” to handle congestion control.
Link Layer Retransmission
Reliability of TCP demands retransmission of lost segments. Depending on the error rate of the wireless channel, the retransmission of lost segments may cause higher BDP (Bandwidth-Delay Product) and bandwidth consumption. If the retransmission can occur in the RLC (radio link control) layer, the smaller size PDUs (packet data units) get retransmitted. This can improve the bandwidth degradation and loss rate (FER—forward error correction) of the communication channel. Unfortunately, this technique suffers from high jitter and BDP (Jonathon Stone and Craig Patridge, “When The CRC and TCP Checksum Disagree. ACM SIGCOMM,” September 2000; (“TCP Over 2.5 G and 3G Wireless Networks Draft-ietf-pilc-2.5g3g-03”.)
The first two techniques above described, Snoop TCP and Wireless Profiled TCP, do not address the deeply routed cause of retransmission. Instead, they focus on how to adapt to the excessive packet loss and retransmission. The Link Layer Retransmission technique attempts to address the root cause, but does not handle it in the best way. Accordingly, among the objects of the present invention are (1) to bring down the packet loss rate; (2) to achieve finer control over retransmission; (3) to achieve sophisticated flow control tuned for a lossy environment; and (4) to enhance the latency, throughput, bandwidth conservation and power savings.